1. Technical Field
The subject matter described here generally relates to wind turbine blades, and, more particularly, to boundary layer fins for a wind turbine blade.
2. Related Art
A wind turbine is a machine for converting the kinetic energy in wind into mechanical energy. If the mechanical energy is used directly by the machinery, such as to pump water or to grind wheat, then the wind turbine may be referred to as a windmill. Similarly, if the mechanical energy is converted to electricity, then the machine may also be referred to as a wind generator or wind power plant.
Wind turbines are typically categorized according to the vertical or horizontal axis about which the blades rotate. One so-called horizontal-axis wind generator is schematically illustrated in FIG. 1 and available from General Electric Company. This particular configuration for a wind turbine 2 includes a tower 4 supporting a nacelle 6 enclosing a drive train 8. The blades 10 are arranged on a “spinner” or hub 9 to form a “rotor” at one end of the drive train 8 outside of the nacelle 6. The rotating blades 10 drive a gearbox 12 connected to an electrical generator 14 at the other end of the drive train 8 arranged inside the nacelle 6 along with a control system 16 that may receive input from an anemometer 18.
The blades 10 generate lift and capture momentum from moving air that is then imparted to the rotor as the blades spin in the “rotor plane.” Each blade 10 is typically secured to the hub 9 at its “root” end, and then “spans” radially “outboard” to a free, “tip” end. The front, or “leading edge,” of the blade 10 connects the forward-most points of the blade that first contact the air. The rear, or “trailing edge,” of the blade 10 is where airflow that has been separated by the leading edge rejoins after passing over the suction and pressure surfaces of the blade. A “chord line” connects the leading and center of trailing edge of the blade. The length of the chord line is simply the “chord.” The thickness of a blade 10 varies across the span, and the term “thickness” is typically used to describe the maximum distance between the low pressure suction surface and the high pressure surface on the opposite side of the blade for any particular chord line.
A “boundary layer” is the zone of reduced velocity air that is immediately adjacent to the surface the moving blade 10. The thickness of the boundary layer is typically defined as the distance from the blade at which the flow velocity is 99% of the “freestream” velocity where the air is unaffected by the viscous or friction forces of the blade, but the potential flow is felt beyond the boundary layer. “Flow separation” occurs when the boundary layer travels far enough against an adverse pressure gradient that the flow velocity speed falls almost to zero. The fluid flow then becomes detached from flowing over the blade 10 and instead forms eddies and vortices.
Such boundary layer separation can increase drag on the blade 10, particularly the “pressure drag” which is caused by the pressure differential between the front and rear surfaces of the object as it travels through the fluid. Boundary layer separation may also lead to stall and vortex shedding that can causes noise and structural vibrations in the blade 10. For this reason much effort and research has gone into the design of aerodynamic surfaces which delay flow separation and keep the local flow attached to the blade 10 for as long as possible. For example, International Patent Publication No. WO 2007/140771 and European Patent Application No. EP 1944505 discloses wind turbine blades with vortex generators. However, such vortex generators may reduce the energy that might otherwise be captured from the wind.